Statement of the Technical Field
The inventive arrangements relate to Light Detection and Ranging (“LIDAR”) systems. More particularly, the inventive arrangements concern systems and implementing methods for modulating an input to a Geiger mode Avalanche PhotoDiode (“APD”) array using Digital Micromirror Devices (“DMD5”).
Description of the Related Art
Conventional LIDAR range finding systems have been used in civil and defense applications for many years. For example, the LIDAR range finding systems have been used to collect 3D information specifying characteristics of terrain and/or objects. In this regard, the LIDAR range finding systems are generally configured to measure the distance therefrom to a terrain surface or object's surface. Such measurements can be achieved using a single laser beam, and time-of-flight computations. A 3D image of the terrain or object surface may then be created using the distance measurements. This type of data is known in the art as 3D cloud point data.
The described LIDAR range finding systems may employ a Geiger mode APD detector or an array of detectors for detecting light reflected off of an object spaced a distance therefrom. The term “Geiger mode” refers to an application of a bias voltage which exceeds the breakdown voltage of the APD detector. When the APD detector is over biased, the APD operates in a metastable state where a single photon may cause an avalanche current. The avalanche current can then be detected using simple digital circuitry. In the case of Lidar, the detection of an avalanche stops a timing circuit which in turn is used to measure the time of flight of a transmitted laser pulse to the object. The distance to the object can then be determined given the speed of light.
Despite the advantages of Geiger mode APD detectors, they suffer from certain drawbacks. For example, the Geiger mode APD arrays may comprise unresponsive detectors (“dead detectors”) and continuously triggering detectors (“hot detectors”). While the data from the dead and hot detectors can be discarded during post processing, the hot detectors may cause crosstalk between surrounding detectors. Additionally, the voltage at which an avalanche occurs (i.e., the breakdown voltage) can exhibit significant variation between detectors in the array. The breakdown voltage level determines the sensitivity or photon detection efficiency across the Geiger mode APD array and cannot be controlled with the high level of precision required for producing superior imagery.
Various techniques have been derived for reducing the undesirable side effects of the variation in detector sensitivity across Geiger mode APD arrays. One technique involves modifying the spacing and geometry of the APDs in the array during design. Another possible technique is to individually adjust the bias level of every APD detector in the Geiger mode APD array. Additionally, a calibration lookup table can be generated for each detector in the array based on the measured sensitivity of the detector. This lookup table can then be used to compensate for the non-uniform array sensitivity during the process. However, such calibration lookup tables do not affect the noise contributions due to hot detectors.